Effects of soft marine fouling on wave-induced forces in floating aquaculture cages: Physical model testing under regular waves

Abstract

Previous experiments on the fouling/current and fouling/wave interactions were mostly limited to two dimensional (2D) net panels or isolated twines. The current paper deals with the wave-induced forces/moments in three dimensional (3D) physical models of a gravity net-cage. The colonial hydroid Ectopleura larynx, a common fouling organism in the aquaculture industry, was artificially modelled and considered in the current experiments. A clean cage and cages with different fouling lengths were tested under deep-water regular wave trains. The time series of the wave-induced forces/moments in the surge, sway, roll and pitch degrees of freedom were recorded and analysed. The experimental results showed that the surge force amplitude increased up to 72% by the fouling, but the sway force amplitude decreased up to 26%. The asymmetry of the surge and sway force waveforms increased by the fouling, respectively. The effects of the fouling on the force/moment response amplitude operators (RAO) were also studied. Pixel histogram analysis was performed to define equivalent net solidities of the fouled nets. In general, the tests under regular waves were found to underestimate the soft fouling effects on the wave-induced forces/moments, as compared to those carried out under irregular waves.